fix(panel): rebalance diff windows after auto-width resize

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR adjusts Diffview panel window resizing so that when an auto-width split panel changes width, the rest of the diff layout rebalances accordingly.

Changes:

• Rebalance window sizes after an auto-width panel resize (Panel:resize()).
• Avoid running wincmd = on FilePanel:open() / FileHistoryPanel:open() when using split + auto width (to prevent redundant/undesired equalization).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lua/diffview/ui/panel.lua	Rebalances windows after an auto-width resize by invoking window equalization before re-rendering.
lua/diffview/scene/views/file_history/file_history_panel.lua	Skips wincmd = during open when split width is auto.
lua/diffview/scene/views/diff/file_panel.lua	Skips wincmd = during open when split width is auto.

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[dlyongemallo]

@copilot I think you may have been wrong about the vim.cmd("vertical wincmd =") change. Adding vertical to this seems to make the entire command do nothing.